It is known that polyurethane and polyurea plastics can be obtained from polyisocyanates and compounds containing active hydrogen atoms. The properties of these plastics vary within a wide range. High strength, elasticity and wear resistance are particularly valuable properties of these products. On the other hand, their thermal stability and, in particular, their permanent dimensional stability at temperatures above 120.degree. C. is only moderate. The use of these products as building and structural components is limited by their inflammability. Although their inflammability can be reduced by the addition of flameproofing agents, their mechanical properties are generally adversely affected.
It is also known that inorganic silica gel plastics can be obtained from aqueous solutions of alkali silicates by the action of (potential) acids. Silica gel plastics have acquired particular usefulness as cements and surface coatings. Lightweight foams have also been produced from waterglass. Products of this kind have a high dimensional stability under heat and are completely non-inflammable. However, they are brittle and have fairly low strength. In the form of foams, they are substantially incapable of withstanding loads and disintegrate under the effect of pressure. It would be extremely desirable to combine the favorable properties of inorganic and organic plastics materials with one another and to eliminate the unfavorable properties of both. Accordingly, there has been no shortage of attempts to produce composite plastics, although to data, no completely satisfactory product has been found.
Polyurethanes have been mixed with active silica as filler and the resulting mixtures subsequently vulcanized. A certain reinforcing effect is obtained in this way, as in cases where highly active carbon black is used. Tensile strength and modulus increase, while elongation at break decreases. The presence of silica does not generally alter the properties of the material. The systems in question are relatively coarse, heterogeneous two-phase systems. The interaction between the two phases is only minimal because of the relatively small interface and the completely different chemical nature of the two phases.
It has also been proposed to use silica in the form of microfibers. In this case, the reinforcing effect is increased by virtue of the specific morphology. On the other hand, the incoherent zones inevitably become larger, so that the chemical interaction between the two phases actually decreases and, thus, the general character of the coarse heterogeneous two-phase plastics material remains intact.
According to U.S. Patent No. 3,607,794, an aqueous solution of an alkali silicate is reacted with a low molecular weight polyisocyanate such as 4,4'-diphenyl methane diisocyanate. The following specific disclosure is made in regard to the isocyanates used:
"Suitable polyisocyanates which be employed in the process of the invention are exemplified by the organic diisocyanates which are compounds of the general formula: EQU O.dbd.C.dbd.N--R--N.dbd.C.dbd.O PA1 where R is a divalent organic radical such as an alkylene, aralkylene or arylene radical. Suitable such radicals may contain, for example, 2 to 20 carbon atoms. PA1 Examples of such diisocyanates are: PA1 Other polyisocyanates, polyisothiocyanates and their derivatives may be equally employed. Fatty diisocyanates are also suitable of the general formula ##STR1## where X + y totals 6 to 22 and z is 0 to 2, e.g. isocyanatostearyl isocyanate. Of the foregoing, p,p'-diphenylmethane diisocyanate has been found in practice to be most suitable. Tolylene diisocyanates, e.g. the 2,4- and 2,6isomers, are also readily available and suitable for use." PA1 "The relative proportions of the alkali metal silicate and the isocyanate may be varied yielding, as noted above, products of different physical characteristics and probably differing chemical structure. PA1 In general it is desirable to employ an excess of the silicate, i.e., a quantity greater than would be stoichiometrically equivalent to the isocyanate employed. On the other hand it is important not to use so little isocyanate that insufficient reaction occurs. Typically, using p,p'-diphenylmethane diisocyanate (which is commercially available at a strength of about 85-90 percent, calculated on a molecular weight of 250), and a sodium silicate of Na.sub.2 O:SiO.sub.2 ratio is 2.0:1.0 to 2.3:1.0, the weight ratio of silicate to isocyanate may vary from 1:7.75 to 3:1." PA1 (1) the need to subject standard commercial-grade polyisocyanates to another, and in some cases, expensive, chemical reaction; PA1 (2) the increased reactivity and increased sensitivity to atmospheric moisture which hydrophilizing generally involves.
p,p'-diphenylmethane diisocyanate (sic) PA2 phenylene diisocyanate PA2 chlorophenylene diisocyanate PA2 tolylene diisocyanate PA2 m-xylylene diisocyanate PA2 benzidene diisocyanate PA2 naphthylene diisocyanate PA2 tetramethylene diisocyanate PA2 pentamethylene diisocyanate PA2 hexamethylene diisocyanate PA2 decamethylene diisocyanate PA2 thiodipropyl diisocyanate
According to the teaching of this patent, commercially available sodium silicate solutions are used as the aqueous alkali silicate solutions. The following disclosure is made in regard to the quantitative ratio of isocyanate to alkali silicate:
The above-mentioned, low-viscosity, substantially bifunctional polyisocyanates are the same as those which are generally used in the polyurethane foam art and with which favorable results are generally obtained in foams. Although readily obtainable, the polyisocyanates described above are only of limited suitability for the production of a composite plastics material based on polyisocyanate and aqueous silicate solutions.
By following the teaching of U.S. Pat. No. 3,607,794, it can be seen that mixtures of aqueous sodium silicate solution and the low-viscosity diphenyl methane diisocyanate only form relatively coarsely divided emulsions. This disadvantage can be offset to some degree by the recommended addition of emulsifiers and foam stabilizers, which make the primary emulsions somewhat more finely divided and more stable, but unfortunately the property spectrum remains unsatisfactory. In particular, the composite plastics obtained are extremely brittle and are very limited in strength. The foams produced in accordance with the known teaching show considerable faults, such as cracks and/or voids, in their foam structure. In some cases, the foam mixture collapses, especially when relatively large quantities are to be foamed. The foam plastics produced from the starting materials described are those only of limited suitability for large-scale manufacture.
According to U.S. application Ser. No. 364,763, filed on 5/29/73 now abandoned, the problems referred to above are solved by using polyisocyanates containing ionic groups which provide for better emulsifiability between organic and inorganic phase, so that inorganic-organic plastics having a better level of properties are obtained by virtue of the more finely divided emulsions.
A similar effect is obtained by modifying the polyisocyanates used with non-ionic hydrophilic groups (see, e.g. U.S. application Ser. No. 469,253, filed 5/13/74.
Unfortunately, the last two processes are attended by the following disadvantages:
U.S. application Ser. No. 446,577, filed 2/27/74, now U.S. Pat. No. 3,983,081, describes a foam concrete based on waterglass, polyisocyanate and inorganic water-binding fillers. The process described in this application generally results in the formation of foam plastics having relatively high gross densities (&gt;100 kg/m.sup.3). In addition, it is necessary, because of the solid filler used in accordance with the invention, to use metering and mixing units of the type normally not used for the production of foams from exclusively liquid components.
The object of the present invention is to obviate the disadvantages of conventional composite plastics referred to above and, in addition, to provide hard inorganic-organic plastics which have the advantage of high toughness and strength and, in the case of foamed materials, which can be produced with fewer difficulties and have a more regular cell structure. More particularly, the object of the invention is to enable high-quality inorganic-organic lightweight foams to be obtained safely and economically from readily available polyisocyanates in standard foaming machines without any need for water-binding fillers to be used.